The present invention relates to a speaker apparatus.
The present application claims priority from Japanese Application No. 2003-051543, the disclosure of which is incorporated herein by reference.
The basic configuration of a conventional dome speaker is such that it has a dome diaphragm B carrying a voice coil A, as shown in FIG. 1A. The conventional speaker further includes a ring-shaped permanent magnet C1, a top yoke C2, and a yoke C3, thus forming a magnetic circuit (an inner magnetic type). The voice coil A is disposed in an annular magnetic gap formed between the top yoke C2 and the yoke C3, while a frame D is provided around the yoke C3, thereby supporting the dome diaphragm B on its edge E.
The dome diaphragm B shown in FIG. 1A has a cross section protruding towards the front side of the speaker. In this way, the overall height of the speaker extends from the bottom of the yoke C3 to the protruding (top) portion of the dome diaphragm B, hence increasing the overall height (thickness) of the speaker by the protruding portion. In order to produce a thin type speaker so as to solve this problem, there has been suggested an inverted dome speaker having a concave cross section formed by protruding the dome diaphragm B towards the voice coil A, as shown in FIG. 1B and as disclosed in Japanese Utility Model Publication No. 54-1242 and Japanese Unexamined Patent Application Publication No. 11-88987.
It is known that the above-mentioned inverted dome speaker generally has a high directivity, and since the dome diaphragm curves continuously with different radiuses, it is allowed to obtain an advantage that high frequency band resonance may be dispersed, thereby increasing a resonance frequency in high frequency band and obtaining a flat sound-pressure frequency characteristic in high frequency band. For this reason, such kind of dome speaker is generally used as a loud-sound playback speaker (tweeter) by taking advantage of such high frequency band characteristic.
Moreover, since the inverted dome speaker mentioned above is formed such that its high frequency band characteristic can be controlled by driving an optimal position of the diaphragm, it is allowed to further improve its directivity as compared with a non-inverted dome speaker having the same diameter. In addition, since a high structural stability and high structural rigidity may be acquired by virtue of the concave cross-section of the diaphragm, it is possible to increase a divisional vibration frequency and to suppress a distortion in a middle-sound band. Besides, by forming the diaphragm having a shallow concave cross section, it is permitted to ensure an improved speaker performance capable of avoiding a front-room-effect (which is a turbulence of a sound-pressure characteristic usually caused by hollow portion on the front side of diaphragm).
In this way, an inverted dome speaker can usually be made thinner in its thickness and more excellent in its high frequency characteristic and directivity than a cone type speaker. However, in order to inhibit a characteristic irregularity caused by a reverse resonance between dome diaphragm and edge, it is desired that edge width (edge outer diameter) be made as small as possible. As a result, a dome diaphragm usually has an effective vibration area located only inside the dome diaphragm. Consequently, various dome speaker apparatuses including such an inverted dome speaker are usually unsuitable for low-frequency band playback. Moreover, since the edge width is narrow, the amplitude range of the diaphragm will be small, thus there has been a problem that this sort of speaker apparatus is not suitable for playback in low frequency band (which usually requires large amplitude).